As a process of forming a copper coating on a resin article as an electrically insulating article, an electroless plating method, a vapor deposition method, a press-contact method, an adhesion by an adhesive, etc., are conventionally known.
The electroless plating method is most general and has been practically used in many technical fields, and is an excellent plating method since low-temperature plating is applicable. However, this plating method usually requires a specific pre-treatment and in the method, there are troublesome problems such as the use of specific chemicals, a long time plating treatment, a waste liquid treatment, etc.
For omitting or simplifying the pre-treatment step, there is a method of using a resin article made of a composition previously compounded with dienic rubber, etc., but it frequently happens that the properties of the resin itself are deteriorated. Further, in the case of resin molding compounded with a reinforcing material such as glass fibers, etc., or a filler, it is difficult to apply a strongly adhesive good plating to the surface of the reinforcing material, which results in causing a disadvantage that the plating is easily peeled off at the portion where the reinforcing material is exposed on the surface of the molding.
As the vapor deposition method, there are known a vacuum vapor deposition method, an ion sputtering method, an ion plating method, etc., and in these methods, a specific equipment such as high-vacuum apparatus is used for the vapor deposition and also a specific pre-treatment is usually required for improving the adhesive property, but there is a disadvantage that the adhesive strength is generally inferior.
Furthermore, a press-contact method, adhesion by an adhesive, etc., can be properly used, but in these cases, the press-adhesion is inapplicable for some articles or there is a restriction on the form of resin articles for which the above method is applicable. In the case of the adhesion method, a previously produced copper foil is used, and therefore, the thickness and size of the copper foil and the kind of the adhesive are restricted and, in particular, the property of the adhesive layer greatly influences on the property of the adhered product.
Still further, it is known that when copper formate is coated on a ceramic and the ceramic is heat-treated in a non-oxidative atmosphere, an article having adhered thereto a copper coating is obtained but in the method, it is difficult to produce articles having strongly adhered thereto a copper coating with good reliability. This method has never been applied to thermoplastic resin or thermosetting resin articles and for these resin articles, the above-described electroless plating method has been mainly used.
A copper clad film prepared by adhering a copper foil onto a film or sheet of a heat-resistant resin such as polyimide resin, etc., has been used, but recently a so-called "double layer film" having a strongly adhered copper coating without using an adhesive inferior in properties such as heat resistance, etc., has been required in the field of electronic materials. However, as described above, a method of forming a copper layer or film on a polyimide film, etc., strongly adhered thereto by a simple and economical manner without using adhesives has not yet been developed.
In the fields of double faced copper clad plates, multi-layer printed wiring boards, etc., having through holes, it has been required to use through holes for interlaminar electro-conducting only using from through holes having a diameter of 0.8 mm or less to small holes having a diameter of 0.35 mm or less from the standpoints of development of small-sized parts and surface mounting techniques.
However, in through hole plating by an electroless plating method conventionally practiced, the plating liquid does not permeate into the inside of small holes having a diameter of about 0.35 mm and fluidity becomes difficult since the electroless plating liquid is a liquid, thereby becoming very difficult to form a uniform copper film or layer on the inside walls of the holes.
In other words, a process of economically forming a copper layer or film having an excellent adhesive strength and a high quality on steric moldings, films, sheets, laminates, etc., of resins by a simple manner has been desired.